Capacitive micromachined ultrasonic transducers generally combine mechanical and electronic components in very small packages. The mechanical and electronic components operate together to transform mechanical energy into electrical energy and vice versa. Because cMUTs are typically very small and have both mechanical and electrical parts, they are commonly referred to as micro-electronic mechanical systems (“MEMS”) devices. cMUTs, due to their miniscule size, can be used in numerous applications in many different technical fields, including medical device technology.
One application for cMUTs within the medical device field is imaging soft tissue. Tissue harmonic imaging has become important in medical ultrasound imaging, because it provides unique information about the imaged tissue. In harmonic imaging, ultrasonic energy is transmitted from an imaging array to tissue at a center frequency (fo) during transmission. This ultrasonic energy interacts with the tissue in a nonlinear fashion, especially at high amplitude levels, and ultrasound energy at higher harmonics of the input frequency, such as 2fo, are generated. These harmonic signals are then received by the imaging array, and an image is formed. To have a good signal to noise ratio during harmonic imaging, ultrasonic transducers in the imaging array would preferably be sensitive around both the fundamental frequency fo and the first harmonic frequency 2fo.
Conventional ultrasonic transducers are not capable of performing in such a manner. For example, piezoelectric transducers are not suitable for harmonic imaging applications because these transducers tend to be efficient only at a fundamental frequency (fo) and its odd harmonics (3fo, 5fo, etc.). To compensate for the odd harmonic efficiencies of piezoelectric transducers, the transducer is typically damped and several matching layers are used to create a broad band (˜90% fractional bandwidth) transducer. This approach, however, requires a trade-off between sensitivity and bandwidth, since significant energy is lost due to the backing and matching layers. Additionally, conventional piezoelectric transducers and fabrication methods do not enable device manufacturers to control or adjust the vibration harmonics of conventional piezoelectric transducers.
Conventional cMUTs are also not generally configured for tissue harmonic imaging. For example, conventional cMUTs are not adapted to and do not utilize the multiple vibration modes of a cMUT membrane. Rather, conventional cMUTs, like conventional piezoelectric transducers, have a substantially uniform circular or rectangular membrane that have only utilized the first vibration mode of the cMUT membrane. In addition, conventional cMUTs and fabrication methods do not provide cMUTs capable of having adjustable vibration modes or controllable vibration harmonics. Due to the design of conventional cMUT types, a 90% fractional bandwidth is usually desired to have a reasonable signal to noise ratio. This fractional bandwidth, however, precludes use of multiple vibration orders of a cMUT membrane for medical imaging applications. Specifically, conventional cMUT designs are not optimized to achieve higher sensitivity over a wide bandwidth or adapted to exploit multiple vibration modes of a cMUT membrane.
Therefore, there is a need in the art for a cMUT fabrication method enabling fabrication of a cMUT with an enhanced membrane to increase and enhance cMUT device performance for tissue harmonic imaging applications.
Additionally, there is a need in the art for fabricating cMUTs to utilize multiple vibration modes and multiple vibration harmonics of a membrane to increase and enhance cMUT device performance.
Additionally, there is a need in the art for a cMUT device capable of receiving and transmitting ultrasonic energy using frequencies associated with different vibration modes for a cMUT membrane.
Still yet, there is a need in the art for a cMUT device having a membrane with vibration modes that are harmonically related.
It is to the provision of such cMUT fabrication and cMUT imaging array fabrication that the embodiments of present invention are primarily directed.